In a differentially pumped mass spectrometer system a sample and carrier gas are introduced to a mass analyser for analysis. One such example is given in FIG. 1. With reference to FIG. 1, in such a system there exists a high vacuum chamber 10 immediately following first and second evacuated interface chambers 12, 14. The first interface chamber 12 is the highest-pressure chamber in the evacuated spectrometer system and may contain an orifice or capillary through which ions are drawn from the ion source into the first interface chamber 12. The second, interface chamber 14 may include ion optics for guiding ions from the first interface chamber 12 into the high vacuum chamber 10. In this example, in use, the first interface chamber 12 is at a pressure of around 1 mbar, the second interface chamber 14 is at a pressure of around 10−3 mbar, and the high vacuum chamber 10 is at a pressure of around 10−5 mbar.
The high vacuum chamber 10 and second interface chamber 14 can be evacuated by means of a compound vacuum pump 16. In this example, the vacuum pump has a first pumping section 18 and a second pumping section 20 each in the form of a set of turbo-molecular stages, and a third pumping section in the form of a Holweck drag mechanism 22; an alternative form of drag mechanism, such as a Siegbahn or Gaede mechanism, could be used instead. Each set of turbo-molecular stages comprises a number (three shown in FIG. 1, although any suitable number could be provided) of rotor 19a, 21a and stator 19b, 21b blade pairs of known angled construction. The Holweck mechanism 22 includes a number (two shown in FIG. 1 although any suitable number could be provided) of rotating cylinders 23a and corresponding annular stators 23b and helical channels in a manner known per se.
In this example, a first pump inlet 24 is connected to the high vacuum chamber 10, and fluid pumped through the inlet 24 passes through both sets of turbo-molecular stages in sequence and the Holweck mechanism 22 and exits the pump via outlet 30. A second pump inlet 26 is connected to the second interface chamber 14, and fluid pumped through the inlet 26 passes through one set of turbo-molecular stages and the Holweck mechanism 22 and exits the pump via outlet 30. In this example, the first interface chamber 12 may be connected to a backing pump (not shown), which may also pump fluid from the outlet 30 of the compound vacuum pump 16. As fluid entering each pump inlet passes through a respective different number of stages before exiting from the pump, the pump 16 is able to provide the required vacuum levels in the chambers 10, 14.
In order to increase system performance, it is desirable to increase the mass flow rate of the sample and gas. For the pump illustrated in FIG. 1, this could be achieved without affecting system pressures by increasing the capacity of the compound vacuum pump 16 by increasing the diameter of the rotors 21a and stators 21b of the turbo-molecular stages of the second pumping section 20. For example, in order to double the capacity of the pump 16, the area of the rotors 21a and stators 21b would be required to double in size. In addition to increasing the overall size of the pump 16, and thus the overall size of the mass spectrometer system, the pump 16 would become more difficult to drive in view of the increased mass acting on the drive shaft 32 due to the larger rotors and stators of the second pumping section 20. Alternatively, if the system flow rate is increased and the pump is not increased in capacity, the pressure at the inlet to the turbomolecular stages, 20, may exceed operational limits. It is a known consequence of this type of turbomolecular technology that operation above approximately 10−3 mbar may cause excessive heat generation and severe performance loss and may even be detrimental to the pump reliability.
It is an aim of at least the preferred embodiments of the present invention to provide a differential pumping, multi port, compound vacuum pump, which can enable the mass flow rate in an evacuated system to be increased specifically where required without significantly increasing the size of the pump.